The prior art is documented with examples of bolt hold open action assemblies in use with the upper and lower receivers of a firearm. The DDLES/Quarter Circle 10 last round hold open linkage, depicted in FIG. 1 of the present application, teaches a version of an elongated linkage which is supported at a single forward end location of the lower receiver and, upon the follower acting upon the linkage, it is caused to pivot about the front pin with the bolt catch being lifted at the rear free extending end. Reference is also made to the Angstadt Arms Disclosure for a lower receiver with bolt catch installation guide.
A further set of examples of last shot bolt hold open assemblies are disclosed in U.S. Pat. No. 8,479,635 and U.S. Pat. No. 9,091,499, both to Overstreet et al., and which each teach a bolt hold open catch assembly configured for operating with rim fire ammunition for use with a firearm including a receiver with an exposed receiving chamber. A drop in housing is supported upon the lower receiving chamber, with a catch supported in elevate-able fashion at an end of the supported housing.
A downwardly extending portion of the catch is configured to being manipulated by an externally configured and toggled bolt catch for raising the housing supported catch into contact with the forwardly traveling bolt, such as following release of a previously retracted charging handle. The catch can alternatively be actuated upwardly by a suitably configured follower associated with an attachable and cartridge holding magazine, this again following a last shot hold open protocol.
Overstreet, U.S. Pat. No. 8,387,296 teaches another version of a bolt catch assembly for use with a firearm including a receiver with an exposed receiving chamber and an underside engageable magazine. The assembly includes a bolt catch actuator housing supported in seating fashion relative to the receiving chamber and communicating with an attachable magazine. A catch actuator is supported in elevatable fashion relative to an end of the actuator housing. A follower component is upwardly and biasingly disposed within the magazine and which, upon contacting the catch actuator, displaces an associated bolt catch of the firearm in an upper and bolt restraining position.
Other examples of displacement or lift engaging bolt catch mechanisms include those taught by Atchisson, U.S. Pat. No. 4,128,042 and Sisgold, U.S. Pat. No. 8,572,875. Also noted is the repeating firearm with bolt-open latch taught by Seecamp, U.S. Pat. No. 3,688,640, and the last shot bolt hold-open device of Osborne, U.S. Pat. No. 4,455,919.